The difference in FSX and FSB is mainly the texture of the micro prism, NOT the size. If you really wanted a larger micro prism, the K-3 would be the correct choice.
FSX
FSB
Ira, now that I've used the the $20 double 45 degree split prism from Ebay/HK for an extended period of time, I would not recommend it. It back focuses by a tiny bit and I have to compensate by having the center slab measure ~.5 mm to the left in order to grab the correct focus.
My K-x's original focusing screen came with a rather thick shim, the $20 ebay version requires a thinner shim, which is a pain in the ass to make out of post-it notes and not advisable for a self-proclaimed spastic. The quality control of the $20 one is also pretty awful, mine came with slight stains that I could not clean off, they're dragging their feet with the refund. The 2 tweezers that came with it were also of poor quality, the plastic one snapped while the metal one has a propensity to scratch anything it touches.
My next choice is the $30 Jinfinance one that people claim to have the correct thickness. Although to be fair the $20 Ebay one has the same thickness as the original Pentax one, at least to the naked eye.
I don't think I can ever justify spending $100+ on Katzeye.
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I've found the split prism to be more useful than the microprism. The microprism has only been useful for objects with extremely fine textures that can't be differentiated easily on the split prism (hair, fur, canvas, etc), even then it's hard to pull the correct focus. The 45 degree or the cross shape EC-L seem to be the best choice for someone with failing vision.
I'm interested in the S TYPE focusing screen, I wonder if the precision is vastly superior to the Pentax screen.